1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus, and more particularly to a heater attachment structure for use in a fusing system of such an apparatus.
2. Description of Related Art
In an electrophotographic image forming apparatus, such as a printer or copier, a latent image is formed on a light sensitive drum and developed with toner. The toner image is then transferred onto media, such as a sheet of paper, and is subsequently passed through a fuser assembly where heat and pressure are applied to melt and adhere the unfused toner to the surface of the media. There are a variety of devices to apply heat and pressure to the media such as radiant fusing, convection fusing, and contact fusing. Contact fusing is the typical approach of choice for a variety of reasons including cost, speed and reliability. Contact fusing systems themselves can be implemented in a variety of manners. For example, a roll fusing system consists of a fuser roll and a backup roll in contact with one another so as to form a nip therebetween, which is under a specified pressure. A heat source may be associated with the fuser roll, backup roll, or both rolls in order to raise the temperature of the rolls to a temperature capable of adhering unfixed toner to a medium. As the medium passes through the nip, the toner is adhered to the medium via the pressure between the rolls and the heat resident in the fusing region (nip).
As an alternative to the roll fusing system, a belt fusing system can be used. In one configuration of a belt fusing system, a heater, such as a ceramic heater may be supported on a supporting member, and a pressure roll may be pressed against the heating element with a thin film belt member therebetween. Heat from the heat source may pass through the thin film to heat and adhere unfixed toner to a medium as the medium is passed between the pressure roll and the belt.
In a known construction of a belt fusing system, a ceramic heater may be attached to a supporting member with an adhesive, such as a silicone adhesive. Assembly of the heater to the supporting member using adhesive generally requires a significant investment in processing equipment, as well as a relatively long “cycle time,” such as twenty-four hours, to allow the adhesive to cure. The adhesive bond may limit thermal expansion of the heater relative to the supporting member where the heater and supporting member have different coefficients of thermal expansion, and thus may cause a structural failure of the heater due to thermally induced stresses. Also, it has been observed that many “late” to “end of life” heater assembly failures have occurred as a result of a failure of the bond formed by the adhesive between the heater and the supporting member. This failure of the adhesive bond may allow the heater to move on the supporting member in an uncontrolled manner in the process direction. With the heater no longer fixed, the thin film belt may track uncontrollably side-to-side in the nip area, which can eventually lead to a failure of the belt and require fuser replacement.
U.S. Pat. No. 5,828,035 discloses a heating apparatus in which a heater is movably supported on a support member without requiring an adhesive to attach the heater. During operation of the heating apparatus, the heater is allowed to move in the process direction within a space defined in a heater holder structure. The disclosed structure permits movement of the heater relative to the nip location, and it is believed that this may adversely affect belt tracking and may lead to additional belt wear and inconsistent print quality as compared to a heater that is precisely fixed in an immovable position.
Further, a wear condition has been observed to occur at certain contact points between the thin film belt and the heater of belt fusing systems incorporating ceramic heaters. In particular, a wear pattern on a glass layer of the ceramic heater contacting the belt has been observed to occur at a location that generally matches up with the ends or outer edges of the belt.
Accordingly, alternative designs of image heating apparatus comprising a thin film belt passing over a heater are desired.